1. Field of the Invention
The present invention relates to a dielectric resonator capable of varying its resonant frequency for use in a microwave or millimeter wave band.
2. Description of the Related Art
A demand for mobile communication systems in 900 MHz and quasi-microwave bands has increased rapidly in recent years and a future deficiency of usable frequencies is therefore apprehended. Systems adapted to multimedia communications such as communication systems for transmitting images or image information are being studied. Such communication systems must be realized as large-capacity high-speed communication systems. The use of millimeter wave frequency bands which are practically unused and in which the band width and the capacity of a communication channel and the communication speed can easily be increased has been taken into consideration.
Conventionally, cavity resonators have generally been used as microwave and millimeter wave band filters for use in oscillators and filters. Recently, however, cylindrical TE.sub.01d mode dielectric resonators have come into wide use in place of high-priced large cavity resonators. In 1975, Wakino et al. made a practical TE.sub.01d mode dielectric resonator of this kind having high stability with respect to temperature by using a temperature-characteristic-compensated dielectric. In general, the temperature characteristics of TE.sub.01d mode dielectric resonators are determined by the temperature characteristics of the material of the resonator. Therefore, TE.sub.01d mode dielectric resonators have the advantage of being free from the need for using an expensive metal such as Kovar or Invar to form the cavity.
Also, variable frequency dielectric resonators have recently been studied for use in voltage controlled oscillators, for example.
FIG. 13 is a perspective view of a conventional variable frequency dielectric resonator constructed by using a TE.sub.01d mode dielectric resonator 301. This variable frequency dielectric resonator consists of a variable frequency microstrip line resonator MR350 having a varactor diode 304, and the TE.sub.01d mode dielectric resonator 301. That is, on an upper surface of a dielectric substrate 306 having a grounding conductor 307 formed on its lower surface, a strip conductor 302 and a strip conductor 303 are formed so that one end of the strip conductor 302 and one end of the strip conductor 303 face each other with a predetermined spacing. The strip conductor 302 and the grounding electrode 307 between which the dielectric substrate 306 is interposed form a microstrip line resonator MR302 while the strip conductor 302 and the grounding electrode 307 between which the dielectric substrate 306 is interposed form a microstrip line resonator MR303. The varactor diode 304 is connected in series between the strip conductors 302 and 303. Thus, the variable frequency microstrip line resonator MR350 is constituted of the microstrip line resonators MR302 and MR303 and the varactor diode 304.
The TE.sub.01d mode dielectric resonator 301 is placed on the upper surface of the dielectric substrate 306 close to the strip conductor 302. The TE.sub.01d mode dielectric resonator 301 and the variable frequency microstrip line resonator MR350 are thereby coupled with each other electromagnetically, thus constructing the conventional variable frequency dielectric resonator constituted of the TE.sub.01d mode dielectric resonator 301 and the variable frequency microstrip line resonator MR350.
The strip conductor 305 formed on the upper surface of the dielectric substrate 306 is placed close to the TE.sub.01d mode dielectric resonator 301, thereby constructing the microstrip line M305 which is constituted of the strip conductor 305 and the grounding conductor 307 with the dielectric substrate 306 interposed therebetween and which is electromagnetically coupled with the variable frequency dielectric resonator.
In the thus-constructed conventional variable frequency dielectric resonator, the resonance frequency is variable by changing the electrostatic capacity of the varactor diode 304. The electrostatic capacity of the varactor diode 304 is changed by changing a reverse bias voltage applied to the varactor diode 304. Also, an external circuit, e.g., a negative resistance circuit or the like can be connected to the resonator through the microstrip line M305.
A variable resonance frequency type of cavity resonator may also be made by providing a varactor diode in a portion of a cavity or by being arranged so that the size of a cavity is changeable.
The conventional variable frequency dielectric resonator constructed by using the TE.sub.01d mode dielectric resonator 301, however, has a complicated structure and is high-priced because the two resonators, i.e., the TE.sub.01d mode dielectric resonator 301 and the variable frequency microstrip line resonator MR350, are used. Also, the resonance frequency of the conventional variable frequency dielectric resonator cannot easily be adjusted. Further, since the conventional variable frequency dielectric resonator is constructed by using the two resonators: the TE.sub.01d mode dielectric resonator 301 and the variable frequency microstrip line resonator MR350, not a simple single mode but two modes, i.e., an even mode and an odd mode, occur. Therefore, if the conventional variable frequency dielectric resonator is used in an oscillator, a mode jump can occur easily from a desired resonance mode to a resonance mode different from the desired resonance mode to cause oscillation at a resonance frequency different from the desired resonance frequency. Also, cavity resonators of the variable resonance frequency type are disadvantageously large in size and high-priced.